The present invention generally relates to a process for producing ultra-fine zeolite crystals and their aggregates, especially relates to the production of zeolite crystal aggregates containing a large quantity of intracrystalline voids whose size is between 1-10 nanometers.
Zeolites are complex crystalline aluminosilicates that form a network of AlO4 and SiO4 tertrahedra linked by shared atoms. Other metals atoms may also be substituted into the framework. The negativity of the tetrahedra is balanced by the inclusion of inorganic cations such as sodium, or organic cations such as tetramethylammonium(TMA) or tetrapropylammonium (TPA), that are present in the synthesis. The interstitial spaces or channels formed by the crystalline network enable zeolites to be used as molecular sieves in separation processes and catalysts for chemical reactions and catalysts carriers in a wide variety of hydrocarbon conversion processes.
Most of the zeolites produced by conventional process are in the form of 0.5-1 micron crystals. An exception is the method recently invented by Otterstedt et al., (U.S. Pat. No. 5,863,516) where colloidal suspension of zeolite with particle size less than 0.2 micron, or 200 nanometers, was produced from a clear silica aluminum solution. Although said pending method claimed that a variety of zeolite could be made into particles smaller than 200 nanometers, no lower bound of the particle size was ever specified. Nevertheless, from the examples provided by the patent documents, it is clearly observed that the minimum average particle diameter obtainable by the said method was 37 nanometers.
Crystal, by definition, is a material with translation symmetry. In other words, atoms are arranged in repeated manner so that the same atom appears at the same location after a crystal is moved a fixed distance. The smallest repeating unit is called a crystallographic unit cell. To be considered as a crystal, a material must contain at least five unit cells in every direction. This is because it takes at least three successful units to confirm the repetition, while the outermost unit cells are under a different environment thus cannot be an exact repetition of the inner ones.
Although there are many kinds of zeolite structures known to date, generally speaking, the smallest dimension of the unit cell in these structures is never much less than 1.0 nanometer. By the above argument, the minimum size of a substance that can be called zeolite should be around 5 nanometers.
If a substance displays the repeated atomic structure of a particular type of zeolite in one direction but not in other directions, it cannot be called a zeolite. For example, Kirschhock et al., and Ravishankar et al., (1-6) have published a method for the preparing of so-called xe2x80x9cMFI zeolitic nanoblocksxe2x80x9d. The nanoblocks they acquired was made of pure silica and exhibited some characteristics of MFI zeolite, but its size did not reach 5 repeating unit cells in one direction, thus cannot be called a zeolite. In fact, not only is the substance unstable under high temperature calcination, but also lacks the hydrophobic characteristic of pure silica MFI zeolite. It is therefore not MFI zeolite.
Thus, there is a need for a process that produces zeolites larger than 5 nanometer in size but smaller than the 30 nanometer colloidal crystals disclosed by U.S. Pat. No. 5,863,516.
A main purpose of this invention is to provide a method for the preparation of ultra-fine zeolite crystals whereas the size of said crystal is between 5-30 nanometers.
Another purpose of this invention is to provide a method for the preparation of zeolitic material as strongly bonded aggregates of said ultra-fine zeolite crystals thus leaving a large amount of intracrystalline voids whose size is between 1 to 10 nanometers.
The benefit of smaller zeolite crystals and large number of intercrystalline void:
Smaller zeolite crystals have more external surface area per mass than larger ones. This is important for catalytic reactions of large molecules that cannot enter the micropores (0.3xcx9c0.9 nm) of zeolite but can react on the external surface of zeolite crystals. The large number of intercrystalline void enables the diffusion of large molecules to the surface of zeolite nano-crystals. If small zeolite crystals are tightly bonded into a dense mass without intercrystalline void, their external surface area will not be accessible to large molecules, and the benefit of small crystal size will be lost. The size of intercrystalline void is not that important in this sense, but smaller void usually related to higher specific surface area of the void. The particle size of the ultra-fine zeolite crystals synthesized with the conditions according to the present invention is larger than 5 unite cells, or about 5 nanometer, but smaller than that of the colloidal zeolites synthesized conditions as Otterstedt et al., or smaller than 30 nanometers. The present invention differs from the international patent WO97/17290, which produces layer type material with zeolite structure in only two-dimension from the exfoliation of precursor for zeolite MCM-22.
This invention discloses a method for the production of ultra-fine zeolite crystals wherein the process comprises of preparing a transparent silicate-containing aqueous solution, which may also contains other components such as Aluminum, Titanium and organic template molecules that is known to produce zeolite; reacting said solution under 25xcx9c100 deg C. for a period of time while the solution remains in transparent without producing colloidal zeolite; adding an anionic surfactant to flocculate the silicates components; drying and forming the flocculated organic/inorganic hybrid mass into shape; heating the shaped mass with water vapor at a temperature of 100-200 deg C. for 1-3 days to produce nanometer zeolite crystals whose crystal size is within 5xcx9c30 nanometers: According to the choice of the process conditions, the product could be loosely bonded zeolite crystals or tightly aggregated crystals containing large amount of intracrystalline voids with size between 1-10 nanometers.